This invention relates to a semiconductor storage device and a memory module, and to techniques which are effective when utilized chiefly in a universal semiconductor storage device such as dynamic RAM (Random Access Memory), and a memory module employing the semiconductor storage device.
An investigation conducted after the completion of the present invention has revealed that there is the official gazette of Japanese Patent Application Laid-open No. 222989/1992 as a document which is thought to be relevant to the present invention to be described later. The technique of the official gazette consists in the contrivance that stressxe2x80x94strain states which appear within a plurality of chips are uniformalized among the chips, the states being ascribable to heat generation during the operation of a semiconductor integrated circuit device in which the chips are sealed in a single package, and an example which realizes a double storage capacity by employing two memory chips is disclosed in paragraph 0031 in the official gazette. The official gazette does not contain any statement to the effect that, as in the invention of the present application to be described later, the thickness (in the direction of the height) of the semiconductor integrated circuit device viewed as one semiconductor storage device is made equal to or less than the thickness of a universal semiconductor storage device having an equal storage capacity.
By the way, memories each having output terminals of 2-bit configurations are stated in the official gazettes of Japanese Patent Applications Laid-open no.20387/1985, No. 130139/1985, No. 171687/1985, No.146495/1987 and No. 262291/1987.
In the case where a plurality of chips are sealed into one package in this manner, the resulting device merely sealed becomes a unique semiconductor storage device, the uses of which are limited. For the purpose of permitting a semiconductor storage device to be widely utilized, it is important to provide this device with compatibility with the existing semiconductor storage devices. According to the semiconductor integrated circuit device in the aforementioned official gazette, the memory chips are held in a stacked structure so that the front surfaces (main surfaces) thereof may be opposite to each other, and a height for wire bonding which executes connections with leads is required over each of the front surfaces of the two chips, so that the package thickens inevitably.
Since packages have a tendency to be more thinned in universal semiconductor storage devices, it is difficult to realize a thickness equal to the thickness of the universal semiconductor storage device having an equal storage capacity by the technique stated in the aforementioned official gazette. In order to realize a large storage capacity within a small volume, each of semiconductor storage devices is often used in the way that memory chips are mounted on both surfaces of a mounting circuit board, thereby to construct a memory module, and that a plurality of such memory modules are plugged into a plurality of connectors which are juxtaposed on a mother board. In this case, the intervals of the individual connectors are set as near as possible. Therefore, the memory modules need to be formed by matching the overall thickness with, at least, the pitches of the connectors. Considering the radiation of heat during a memory operation, however, the memory modules should desirably be spaced from one another even if slightly.
As a result of the study stated above, the inventors of the present application have found it very important that, when two memory chips are mounted in a single package so as to realize a double storage capacity, the thickness of such a package is made equal to or less than the thickness of an existing semiconductor storage device in which a single memory chip is mounted.
An object of this invention is to provide a semiconductor storage device which can be formed thin while employing two memory chips. Another object of this invention is to provide a semiconductor storage device which has an excellent universality while employing two memory chips. Still another object of this invention is to provide a memory module which is capable of increasing the storage capacity per unit volume and high-density packaging. A further object of this invention is to provide a memory module which is well adapted for use. The above and other objects and novel features of this invention will become apparent from the description of this specification and the accompanying drawings.
A typical one of inventions disclosed in the present application is briefly summarized as follows: Two memory chips each being subjected to memory accesses in 2-bit units are assembled into a stacked structure by placing their back surfaces one over the other, so as to make memory accesses in 4-bit units.
Another typical one of the inventions disclosed in the present application is briefly summarized as follows: A memory module is so constructed that a plurality of semiconductor storage device, in each of which two memory chips each being subjected to memory accesses in 2-bit units are assembled into a stacked structure by placing their back surfaces one over the other, so as to make memory accesses in 4-bit units, are mounted on a mounting circuit board which is square and formed with electrodes (connectors) along one latus thereof.